The present invention relates to the processing of the signal in spread spectrum digital radio communication receivers.
The invention finds application in the field of code-division multiple access (CDMA) radio communications, such as UMTS (“Universal Mobile Telecommunication System”).
In a CDMA system, the symbols transmitted, binary (±1) or quaternary (±1±j), are multiplied by spreading codes composed of samples, called “chips”, whose rate (3.84 Mchip/s in the case of UMTS) is greater than that of the symbols transmitted. Orthogonal or quasi-orthogonal spreading codes are allotted to various logical channels sharing the same carrier frequency, so as to allow each receiver to detect the sequence of symbols which is destined therefor, by multiplying the signal received by the conjugate of the corresponding spreading code possibly shifted to compensate for the propagation times.
An advantage of the CDMA system is that it makes it possible to utilize radio propagation multipaths, by using what is referred to as a rake receiver.
The rake receiver performs coherent demodulation based on an approximation of the impulse response of the radio propagation channel by a series of spikes, each spike appearing with a delay corresponding to the propagation time along a particular path and having a complex amplitude corresponding to the attenuation and to the phase shift of the signal along this path (instantaneous realization of fading). By analyzing several reception paths, that is to say sampling, several times, the output from a filter matched to the spreading code of the channel, with delays corresponding respectively to these paths, the rake receiver obtains multiple estimates of the symbols transmitted, which are combined to obtain a diversity gain. Combining can be performed in particular according to the so-called MRC (Maximum Ratio Combining) method, which weights the various estimates as a function of the complex amplitudes observed for the various paths.
In order to allow this coherent demodulation, a pilot channel can be provided for the estimation of the impulse response in the form of a succession of spikes. The impulse response is estimated by means of a filter matched to a pilot spreading code with which the sender modulates a sequence of known symbols, for example symbols set to 1. The positions of the maxima of the output from this matched filter give the delays used in the fingers of the rake receiver, and the associated complex amplitudes correspond to the values of these maxima.
The rake receiver comprises a set of fingers making it possible to process one and the same sequence of information symbols that is received along different propagation paths from the sender. These paths are usually defined as delays with respect to a reference time.
In a representation of the CDMA signal received, which representation is utilized in a conventional rake receiver, this signal x(t) may be written:
                              x          ⁡                      (            t            )                          =                                            ∑              k                        ⁢                                          ∑                i                            ⁢                                                ∑                  n                                ⁢                                                                                                    A                        i                        k                                            ⁡                                              (                        t                        )                                                              ·                                          b                                              E                        ⁡                                                  [                                                      n                            /                                                          Q                              k                                                                                ]                                                                    k                                        ·                                          c                      n                      k                                                        ⁢                                      p                    ⁡                                          (                                              t                        -                        nT                        -                                                  τ                          i                          k                                                                    )                                                                                                    +                      ɛ            ⁡                          (              t              )                                                          (        1        )            where E[.] designates the integer part operator, k is an index of the CDMA channels superimposed on the relevant carrier, i is an index of the propagation paths existing between the sender or senders and the receiver, n is an index of the chips, p(.) designates the modulation shaping function (“pulse”), the vector (bE[n/Qk]k) is the symbol string shaped by the sender on channel k, Qk is the spreading factor of the k-th user, the vector (cnk) is the spreading code of channel k, τik is the propagation delay for path i and sender k, Aik(t) is the complex amplitude along path i for user k at the instant t, and ε(t) is additive noise.
The number of fingers of a rake receiver is generally limited. By way of example, a base station or a mobile terminal receiver using CDMA access technology typically possesses from 4 to 8 fingers per channel to be processed. This translates into a limitation in the number F of fingers of a rake receiver per user, which is frequently less than the number N of paths observed on the propagation profile existing between the sender or senders and the receiver for this user. The receiver is therefore required to select F paths from among the N paths observed, it being possible to do this according to various strategies. These selection strategies differ from one another in that each of them represents a compromise between complexity, and consequently consumption of the hardware and software resources in the receiver, and performance in terms of mean bit error probability after combination.
An optimal use of the resources of the rake receiver seeks the best selection of the paths available having regard to the number of fingers available to process them. An object of the present invention is to propose a mechanism which achieves a good compromise between the performance available according to various strategies for selecting paths and the computational cost engendered by the operations for processing the signal in the receiver.